The marking of ceramic materials, glazes and glasses can be effected by conventional marking and decoration methods such as etching, cutting, engraving, grinding or by applying a glass or glaze colorant. In these methods, the surface of the marked material is altered with the consequence that the material may suffer damage, especially if marking is effected by etching, engraving, or cutting. The application of a glass or glaze colorant necessitates, in addition, a second firing step. The markings so produced are not always satisfactory in all respects.
It is also known to mark glass by means of a laser beam, whereas the known methods are based on melting or removing substrate material such that the surface of the marked material is also altered.
German Offenlegungsschrift 3 539 047 postulates a method of decorating, marking, and engraving enameled objects using laser beams by incorporating into the enamel coating opacifying agents which the laser beam causes to decompose optically and locally; for example, oxides of titanium, tin, cerium, and antimony. A drawback of this method is that, for example, transparent enameled objects cannot be marked because the opacifying agent incorporated in the enamel coating does not change optically at the non-irradiated areas and, therefore, strongly influences the overall appearance of the object. Furthermore, the opacifying agent employed may adversely affect the mechanical properties of the enamel.
Industry has sought to surface mark glass, ceramic, porcelain, metal, plastics, and the like with four physical attributes. These four attributes are high-resolution, high-contrast, permanence, and speed.
Well known efforts to date have only produced two or three of these attributes. For example, kiln marking ceramics using glass frit material at kiln temperatures ranging from 100° to 1000° C. results in high-resolution, high-contrast, permanent indicia on ceramics, glass, and metals. These known processes require heating the entire substrate along with the glass frit or metal oxide marking material in a kiln. The problem with these processes is the time factor and energy consumption are not commercially efficient to create the indicia. Time factors ranging from minutes to hours are common. Energy consumption of a kiln is generally measured in kilowatts per ton and/or BTUs per pound. Furthermore, these processes do not lend themselves to portability.
Another known marking method is peening on metal. This method cannot be used on glass, ceramic, or other brittle materials because of surface damage and/or breakage. Where used, this method produces a high-resolution, permanent, fast surface indicia. However, high contrast marks are not produced.
Other known marking methods are ink printing methods. One state of the art transfer printing method is taught by WO 95/13195 (May 1995) to Meneghine et al, assigned to Markem Corporation. These methods use a laser-transferable ink on a plastic carrier. The ink is mixed in a transfer medium solution in order to enhance the conversion of laser (IR) energy to heat. These methods produce a high-resolution, high-contrast, and relatively fast method. There is a UV cure step which is time consuming. The problem with this and all ink methods is a lack of permanence. Acids and other solvents remove ink from a hard surface. This method teaches curing the ink onto the substrate surface. The present invention teaches bonding a marking medium to form a new marking layer atop the substrate surface rather than transferring an ink to the substrate and then curing the ink.
Another well known marking method teaches the use of ink jet printers. In order to improve application performance, appearance and permanence, environmentally hazardous solvents are mixed with the ink. Even with these hazardous solvents however, significant improvement has not been achieved.
U.S. Pat. No. 4,541,340 (1985) to Peart et al. discloses a printing process for marking fabrics or plastics in a permanent image. Sublimable dyes are used such as nitroso dyes. A diffusion of the dyestuff into the substrate is caused by a pressurized air step on a transfer label. Only application to fabrics and plastics is taught. The chemistry is different from the present invention. However, the result of a permanent high contrast mark is claimed.
Another related group of marking methods is laser combined with glass frit or metal oxide marking media. U.S. Pat. No. 4,769,310 (1988) to Gugger et al. teaches first creating a glaze in a kiln process. The glaze has a radiation sensitive additive in amounts ranging from 0.01 to 30% by weight. This glaze is then irradiated by a beam of Nd:YAG pulsed laser having light pulses of six to eight nanoseconds at a wavelength of 0.532 μm and a pulse content of 250 milli-joules. The problem with this method is the burden of creating a time consuming glaze surface before applying the high-speed laser beam.
U.S. Pat. No. 5,030,551 (1991) to Herren et al. teaches a laser-based method to mark ceramic materials, glazes, glass ceramics, and glasses by first applying to a workpiece a 100 to 10,000 Angstrom thick transparent layer of titanium dioxide. Second, the workpiece is fired in an oven at 620° C. for one minute and then slowly cooled in the closed oven. Third, the layer is irradiated with a pulsed laser in accordance with the form of the marking to be applied. The laser light must have a wavelength which is sufficiently absorbed by the oxide layer so that a discoloration of the oxide layer is produced at the irradiated areas. The problem with this method is the time and energy-consuming step of firing and cooling the workpiece.
The method of the present invention makes it possible to produce a direct and rapid marking that is indelible and which is, therefore, abrasion and scratch-proof. The markings obtained are also corrosion-proof, solvent-resistant, dimensionally stable, free from deformation, fast to light, heat, and weathering, easily legible, and have good contrast and very good edge definition. In addition, there is virtually no impairment of the mechanical, physical, and chemical properties of the marked material, e.g. mechanical strength and chemical resistance.
There has now been found a flexible method which makes it possible to mark metals, plastics, ceramic materials, glazes, glass ceramics and glasses without damaging the surface thereof and without specific requirements being made of the substrate, which method comprises the use of a glass frit based or mixed organic materials or mixed metal oxide layer for the laser marking.
Accordingly, the present invention relates to a method of radiantly marking both conductive and dielectric materials including metals, plastics, ceramic materials, glazes, glass ceramics and glasses of any desired form which comprises steps of applying to the substrate material a marking material which, depending upon its principal components, may or may not contain at least one energy absorbing enhancer, then irradiating said marking material layer with a laser or diode based energy source such that the radiation is directed onto said layer in accordance with the form of the marking to be applied, and using laser or diode based energy of a wavelength which is sufficiently absorbed by the marking material so that a bonding occurs on the substrate, thereby forming a marking layer atop the substrate.
A preferred embodiment of the present invention employs electrostatic methods of applying marking materials to the substrates. The principles behind electrostatics have been applied in the development of electrophoresis, powder coating sprayers, xerography and ink jet printers. Electrostatic coating technology has been available for many years and is widely used for the coating of household appliances such as ranges, refrigerators, washing machines and dryers. There are some subtle aspects to this science such as fine atomizing of liquid droplets, fine de-agglomerating and diffusion of powders, eliminating or shielding unintended target areas, creation of an optimal electrical charge on the part surface as well as optimizing part geometry and orientation. Characteristics of electrostatic coating processes include low energy expenditure, absence of pollution or other undesirable effluents, and high material utilization efficiencies. Its applications reduce waste and improve manufacturing efficiency and product quality. There are no apparent adverse secondary effects from application of electrostatics.
Behind the operation of all electrostatic coating equipment is the fundamental principle that oppositely charged bodies attract one another. Therefore, charged marking material particles would be attracted towards a grounded or oppositely-charged article. In the electrostatic coating process, the target substrate is grounded so that it is electrically neutral. The coating system creates, electrically charges, and disperses solid particles or liquid droplets of the marking material toward the target substrate by a variety of methods well known to one skilled in the art. The charged marking material particles are attracted to the grounded, neutral substrate and are deposited on it. Since the charged particles are all charged alike they repel from each other during the flight to the target and while “landing”. These marking material particles avoid each other and seek areas on the target surface that are best grounded (i.e. uncoated areas). This is one of the simplest and most elegant aspects of the electrostatic coating process: deposition is uniform because the least coated areas get coated by the “newest” particles. The use of electrostatic deposition technology means that very good reproducibility and precision of deposition can be obtained—relative standard deviations (RSDs) of 1-2 percent of coating thickness can be achieved. This is a significant improvement and tremendous advantage over conventional coating methods.
Since the article being coated is the collecting electrode in the electrostatic coating process, it should have sufficient electrical conductivity, either through its bulk or across its surface, to carry away the electrical charge arriving on the surface with the accumulating marking material particles. For this reason, the electrostatic coating process is most often used to coat objects which are natural conductors of electricity (e.g. metals).
Typically, such conductive articles are held at a grounded potential by merely being supported from a grounded conveyor with a metal hook. By induction from the charging electrode, the conductive article assumes an electrical charge, which is opposite to that of the charged marking material particles. Accordingly, the electrically conductive article attracts the charged marking material particles.
Notwithstanding the above, electrostatic coating practices are also used to coat articles made from non-conductive or dielectric materials (e.g. plastics, glass, ceramics, wood, etc.), hereinafter collectively referred to as “dielectric materials”. When used for these purposes, it becomes necessary to make the dielectric material either a permanent or temporary electrical conductor. A number of techniques have been perfected to accomplish this objective and these methods are well known to one skilled in the art.
For example, molded rubber steering wheels are not natural conductors of electricity; however, they can be made electrically conductive by heating them to temperatures of at least about 212° F. (1000° C.). While this practice works well for electrostatically coating some dielectric materials, it has a number of problems associated therewith. For example, this practice cannot be used to induce a charge on those dielectric materials which do not become electrically conductive when heated (e.g., wood). Moreover, this practice also cannot be used to induce a charge on those dielectric materials, which begin to deform or degrade at or below the temperature needed to make them electrically conductive.
Another method of electrostatically spraying a dielectric material consists of coating the material with an electrically conductive primer. This practice is used in the coating of toilet seats. Specifically, toilet seats are normally made from a phenolic resin/wood-flour mixture. This material is non-conductive and does not become conductive upon heating. Accordingly, to make it possible to electrostatically coat these items, the seats are first sprayed with an electrically conductive, film forming primer. When dried, this coating creates an electrically conductive film on the surface of the seat. After being coated with this primer, the seats are supported from a grounded conveyor with metal hooks. Thereafter, the marking materials could be electrostatically applied.
Electrostatic coating methods are disclosed in many patents.
U.S. Pat. No. 2,622,833 discloses a process and apparatus for electrostatically coating the exterior surfaces of hollow articles made from a dielectric or non-conductive material without the use of backing electrodes, which conform to the shape of the article. In that patent, the articles being coated are mounted onto spindles, which are connected to a conveyor system. The conveyor and the spindles are electrically conductive. Moreover, they are both connected through a conductor to either a ground or a power supply.
In U.S. Pat. No. 2,622,833, a conductive probe, which has an ionizing point or points, is electrically connected to the spindles. This probe is positioned so that it passes, through the article's opening, into the cavity of the article being coated. The spindles then carry these articles between oppositely disposed, spaced negatively-charged electrodes. As the articles pass the electrodes, an electrostatic field is created between the negatively-charged electrodes and the exterior surface of the article. One or more spray guns are directed so as to introduce an atomized coating composition in a direction generally parallel to the path of travel of the articles into the space between the articles and the electrodes. As the marking material particles enter into the ionizing zone, they accept a negative charge and are thus drawn to the grounded or positively-charged article.
U.S. Pat. No. 4,099,486 also discloses a process and apparatus for electrostatically coating glass bottles by using a particular chuck for supporting the bottles which is designed to prevent build-up of coatings thereon. That patent induces a charge onto the glass bottles by heating them to a temperature ranging between 150° F. (66° C.) to 450° F. (232° C.). According to U.S. Pat. No. 4,099,486, the supporting chuck is made from a non-conductive plastic. This chuck fits over a grounding plug, which is designed to ground the bottle by being in physical contact therewith. For example, one embodiment of a ground plug described in that patent is in the form of a flat-headed probe upon which rests the neck of the bottle. Another embodiment of a ground plug described in that patent is in the form of a flat-ended rod which extends into the bottle's opening, and through the bottle's entire length, until the distal end of the rod contacts the inside surface of the bottle's base. Yet another embodiment of a ground plug described in that patent is in the form of a flat-ended rod whose outside dimension is parallel to the inside dimension of the bottle's opening. With this latter configuration, when the ground plug is inserted into the bottle's opening, the outside walls of the plug contact the inside walls of the bottle's neck. Additional patents disclosing electrostatic coating methods and apparatus include:
U.S. Pat. No. 6,063,194 (Dry Powder Deposition Apparatus)
U.S. Pat. No. 5,830,274 (Electrostatic Deposition of Charged Coating Particles onto a Dielectric Substrate)
U.S. Pat. No. 5,698,269 (Electrostatic Deposition of Charged Coating Particles onto a Dielectric Substrate)
U.S. Pat. No. 4,099,486 (Electrostatically Coating Hollow Glass Articles)
U.S. Pat. No. 4,110,486 (Electrostatic Powder Coating Method)
U.S. Pat. No. 3,930,062 (Composition and Method for Electrostatic Deposition of Dry Porcelain Enamel Frit)
U.S. Pat. No. 3,558,052 (Method and Apparatus for Spraying Electrostatic Dry Powder).
Fully integrated electrostatic coating systems are commercially available for efficient coating of small parts in laboratory and batch-production operations from companies such as Trutec Industries, Powder Spray Technologies, Double D Equipment Company and Wagner International. To coat small parts, use of an electrostatic or hot-dip fluidized bed system can provide efficient coating quality. An electrostatic fluidized bed can be used for either electrostatic deposition or for hot-dip coating of small parts, and can operate with just a few ounces of marking materials.